32-2 Chemical Observations upon Spathic Iron. 



But it remains to ascertain what change takes place by 

 the operation of the air on refractory ores, as well when not 

 subjected to the operation of roasting as when they have 

 been subjected : here I can only offer conjectures ; and I can 

 only support them by facts formerly known, and by no 

 direct experiments. 



It is well known that spathic iron ores, exposed to the air 

 and to humidity, become brown, and lose their hardness. 

 Their cohesion is almost destroyed. This effect is produced 

 by the oxidation of the metals. The magnesia, which in 

 the ore is combined with carbonic acid, preserves this acid, 

 while its combination with the metals is destroyed by the 

 stronger oxygenation of the latter : thus it ought easily to be 

 carried off by rain, because we know that the carbonate of 

 magnesia is a little soluble ; and we should think that during 

 an exposure of sixty years this earthy salt might be carried 

 off almost entirely. Its solubility may, besides, be augmented 

 by the carbonic acid united to the metals, and which is se- 

 parated from them in proportion as they oxidize. 



After roasting, the same explanation will not be applica- 

 ble ; in fact, magnesia should not retain carbonic acid any 

 longer, and it is doubtful that it could have regained enough 

 of it In the atmosphere to become again soluble; but as al- 

 most all the spathic ores which have been submitted to this 

 operation contained pyrites, it appears to me that we may 

 explain their amelioration by exposure to the air, by taking 

 in the action exercised on the caustic magnesia which is 

 formed in the mineral by the sulphate of iron. The expe- 

 riment of Bayen will be recollected, which I have mentioned, 

 and by which he thought he had separated the oxide of zinc 

 from the calcined ore, with the assistance of green vitriol. 

 This action certainly takes effect in the places where the 

 roasted mineral is deposited ; and the water, by degrees, 

 takes off the sulphate of magnesia*. 



If this cause should appear insufficient, I would observe, 



# If there is lime in the ore, and it should happen to be in the caustic state, 

 it would decompose the sulphate of iron in preference; but the experiment 

 of Darcet indicates that the salt which would be formed would contribute to 

 the fusion of magnesia. 



that 



